1. Field of the Invention
The present invention relates to a lighting control device and a control method. In particular, the present invention relates to a lighting control device and a lighting control method suitable for use in controlling an amount of light from a headlamp of a vehicle, such as an automobile.
2. Description of the Related Art
Generally, a technology is known in which glare experienced by an oncoming vehicle, a leading vehicle, a pedestrian, and the like is suppressed without compromising peripheral visibility by light from a headlamp being dimmed. A device that can extend battery life and headlamp life by dimming the light from the headlamp and suppressing unnecessary power consumption is also proposed. As such conventional technologies, for example, a method is proposed in which a headlamp is controlled using a predetermined pulse width modulation (PWM) duty ratio corresponding to a requested fade rate (such as Japanese Patent Laid-Open (Kokai) Publication No. 10-029462).
FIG. 8 is a block diagram of a configuration of a lighting control device according to the conventional technology.
In the diagram, a central processing unit (CPU) 1 determines an appropriate fade rate based on a state of each switch 10 to 12 and a signal from a sensor 13. The CPU 1 performs PWM control on a gate voltage of a field-effect transistor (FET) 2 using a predetermined duty ratio S, thereby controlling a drive current sent to a headlamp HL. The predetermined duty ratio S corresponds with the fade rate. As a result, the CPU 1 controls the luminance of the headlamp HL. The CPU 1 can control the luminance of the headlamp HL using only the duty ratio S, without changing the gate voltage value of the FET 2. Therefore, PWM control is very efficient compared to a rheostatic control method. PWM control is a method that is generally used today.
Control can be easily performed if correspondence between the requested fade rate and the PWM duty ratio when the control is actually performed has a correlation that can be expressed by a linear function. Therefore, a technology is known in which control is performed using a duty ratio that has been changed to fulfill values provided as targets (current value, luminance, etc.). Such technology is, for example, disclosed in Japanese Patent No. 3061050, Japanese Patent No. 3063755, and Japanese Patent Laid-Open (Kokai) Publication No. 11-126696.
However, in the above-described conventional technology (Japanese Patent Laid-Open (Kokai) Publication No. 10-029462), the relationship between the requested fade rate and the PWM duty ratio when the control is actually performed is unclear.
Correction taking into consideration fluctuations in the power supply voltage, wiring resistance, and the like, and a difference in the electrical current between a left headlamp and a right headlamp is not made. Therefore, the error in the luminance of the headlamp actually being controlled increases with regard to the requested fade rate. Moreover, a difference in luminance between the left headlamp and the right headlamp occurs.
As a result of individual characteristics (resistance value, etc.) of the headlamps, wiring, and the like being used, the fade rate and the duty ratio often have a non-linear relationship or a correlation that is difficult to mathmatize. However, in the above-described conventional technologies (Japanese Patent No. 3061050, Japanese Patent No. 3063755, and Japanese Patent Laid-open Publication No. 11-126696), control is performed under a presumption that the correlation is linear. Therefore, control under a presumption that the correlation is nonlinear is impossible to attain.